1. Field of the Invention
The proposed invention is related to the field of structural vibration and acoustic (noise) control.
2. Description of the Prior Art
A number of engineering applications can be described as a flexible structure surrounding a cavity. In these systems, structural vibrations induced by either force inputs or external acoustic pressure loads produce sound (or noise) within the cavity. Some common examples of acoustic cavities enclosed by a flexible structure include airplanes, trains, cars and spacecraft launch vehicles. A subset of this group of applications is civil structures or buildings, where relatively rigid walls are combined with flexible windows and other panels.
For the case of spacecraft launch vehicle fairings, the structure is typically constructed from a stiff composite material. The launch vehicle fairing is subjected to extremely high levels of structural vibration and noise at launch. This vibration and noise can damage delicate payloads. The acoustic response of the volume enclosed by the flexible composite structure is dominated at low frequency by very lightly-damped structural acoustic modes (50 Hz-250 Hz). In spacecraft applications, mass and volume are critical parameters, therefore there is a budget on how much noise treatment can be added to the fairing in order to mitigate noise and vibration. Passive methods for attenuating low-frequency disturbances such as foam linings and acoustic blankets are not effective at low frequency. Furthermore, they provide negligible structural vibration attenuation.
Active control strategies have been applied to reduce noise inside acoustic cavities such as aircraft fuselages and automobiles, and have demonstrated significant success in coupling to and attenuating low-frequency acoustic modes. (Fuller, C. R., et. al., "Experiments on Reduction of Aircraft Interior Noise Using Active Control of Fuselage Vibrations," J. Acoust. Soc. Am., 78(S1), S79, 1985; Fuller, C. R., et. al., "Active Control of Sound Transmission/Radiation from Elastic Plates by Vibrational Inputs," J. Sound and Vibration, 136(1), pp. 1-15, 1990). However, these strategies have the disadvantage of requiring complex control algorithms, digital signal processing hardware, power amplifiers, signal conditioning, and extensive cabling, which greatly increases the mass of the system. In addition, active acoustic control techniques do little to reduce the vibration of the structure or to prevent noise from being transmitted through the structure. Finally these techniques have never been demonstrated at acoustic levels commensurate with space launch.
There are several accepted methods of reducing noise transmission from external sources into a cavity interior. Most methods are designed to reduce structural vibration by increasing the mass, stiffness or damping of the structure. Traditional, localized, reactive vibration-suppression devices such as vibration absorbers and tuned mass-dampers are very effective at increasing localized structural impedance and structural damping, respectively. (Bies, D., and Hansen, C., Engineering Noise Control, Theory and Practice, E&FN SPON, 2.sup.nd edition, NY, 1996). The disadvantage of such devices is the necessity of additional mass for their operation. Furthermore, these devices act only on the structure, and do little to attenuate the acoustic dynamics of the cavity.
Recently, work has been done to combine active structural control with active acoustic control. (Jolly et. al., Hybrid Active-Passive Noise and Vibration Control System for Aircraft, U.S. Pat. No. 5,845,236, 1998; Fuller, C. R., Apparatus and Method for Global Noise Reduction, U.S. Pat. No. 4,715,559, 1987; Hodgson, et. al., Broadband Noise and Vibration Reduction, U.S. Pat. No. 5,526,292, 1996; Majeed et. al., Active Vibration Control System for Attenuation Engine Generated Vibrations in a Vehicle, U.S. Pat. No. 5,332,061, 1994). These methods utilize arrays of structural sensors such as accelerometers, and acoustic sensors such as microphones to sense disturbances and actively cancel them. Typically in these control systems, loudspeakers are driven by a control signal 180.degree. out-of-phase with the sensed disturbance to provide cancellation. Also, active vibration absorbers, passive vibration absorbers, or structural actuators such as proof-mass actuators, piezoceramic actuators, or shakers, are used by the control scheme to control structural vibration. Although these active methods have been successful at reducing structural vibration and interior noise, they require a considerable degree of sophistication and hardware to implement. The mass of the hardware (including actuators, sensors, controllers, signal conditioning hardware, power amplifiers, mounting apparatus, and cabling) can become prohibitively large and negate the value of using such systems. Furthermore, complex control systems such as these have a greater chance of component failure, which can be catastrophic in critical applications.